It's not. Free speech is to prevent a state encroaching on or involving themselves in the censorship of citizens.
Social media operators can do what they want in terms of censorship. It's their's to censor and if someone doesn't like it they may simply choose not to use it. It does not impact their wellbeing or freedoms within the state and is not a social service.
Edit: So quite ironically, suggesting limitations on 1A is against the 1A. It is there to prevent limitations of freedoms by things the state may try to impose, should a state ever become corrupted.